1. Field of the Invention
This invention relates to structural resins and in particular to the process of forming solid polyamic acid and polyimide fibers by wet spinning, whereby fibers with excellent chemical resistance, high thermal stability, and tensile properties in the range of standard textile fibers are produced.
2. Description of the Related Art
Linear aromatic polyimides are finding increased usage in industrial and aerospace applications due to their excellent chemical resistance and high temperature stability. They are mainly used in film form, as coatings and composite matrix resin. Various patents and articles have described the formation of aromatic polyamic acid and polyimide fibers, but little commercial development has resulted. Recently, Lenzig AG reported the production of a commercially available aromatic copolyimide fiber P84 using a special dry spinning and finishing process (Proc. 2nd Inter. Conf. Polyimides 1985, 253-271). The main advantages of P84 compared to other high performance fibers are reportedly its outstanding non-flammability, long term thermal stability, non-melting behavior, and excellent chemical resistance to acids and organic solvents. These properties are common to most aromatic polyimides. Suggested applications for this type of fiber are protective clothing, sealing materials, filtration in harsh chemical and/or high thermal environments, and various other textile uses where fire-resistant properties are required.
Production of aromatic polyamic acid fibers by the extrusion of a polyamic acid resin solution into a liquid coagulation medium was reported (U.S. Pat. No. 3,179,614) as early as 1965. The aromatic polyamic acid is generally formed in aprotic organic solvents such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethylsulfoxide (DMSO), and N-methylpyrrolidione (NMP) at concentrations of between 0.05 and 40% solids (w/w). Resin inherent viscosities were found to vary from 0.1 to 5.0 dl/g. Mono-, di-, or trihydric alcohols, or mixtures thereof, or aqueous solutions, or acetone solutions of said alcohols, aqueous solutions of aprotic organic solvents, and thiocyanate or sulfur salts in aqueous DMAc have been used as coagulation media. No disclosure has been found of the production of totally void free solid aromatic polyamic acid fibers that do not contain macropores or voids.